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EHU researchers have explored how a sustainable urban drainage system built in Legazpi has affected various rainwater parameters, and have concluded that a permeable pavement has improved the quality of runoff water and has reduced turbidity, suspended solids and the amount of certain metals.

They also found that once this rainwater has been filtered and its biological quality ensured, it can be used for washing and/or irrigation purposes. The work is in the Journal of Hydrology.

When rainwater reaches the ground in cities, it flows directly into the sea or into rivers or else gets mixed up with the dirty water that flows along the sewers into the water treatment plants. This can lead to various problems, such as the pollution of seas and rivers and a large increase in the amount of water to be treated in the treatment plants during downpours. Urban pavements do not generally allow the water to infiltrate and completely hamper the natural process of the water, but the use of permeable pavements is expanding, albeit very gradually.

Sustainable urban drainage systems are designed to protect the natural water cycle, manage flood risks and improve the quality of runoff water. A combined system has been built in Legazpi: "In one of the town's car parks there is a permeable pavement where the infiltrated and filtered water is collected in a rainwater tank," explained Eneko Madrazo-Uribeetxebarria, a lecturer at the EHU's Faculty of Engineering—Gipuzkoa.

On the whole, permeable pavements imitate the natural water cycle: "Rainwater penetrates the pavement, and thanks to the pavement's pores and gravity, it is infiltrated into natural soil. The rainwater is also filtered through the pores along this infiltration path. What is more, this process is slow, so it means that some of this water may evaporate," said Ainhoa Lekuona-Orkaizagirre, a Ph.D. student at the Faculty of Chemistry.

At the request of Legazpi City Council, researchers from the EHU-University of the Basque Country studied the impact of this system on the quality of runoff water. More pollutants were studied than in any other studies to date.

After comparing the water that has passed through an ordinary asphalt pavement with the water that has passed through the permeable pavement and the water collected in the tank, they "found that multiple parameters are better in the water that has passed through the permeable pavement.

"The quality of the runoff water is better; the turbidity of the water and the amount of suspended solids are significantly lower; and the concentrations of certain metals, such as iron, manganese, vanadium and copper, are also lower," stated Lekuona.

As the researcher explained, turbidity is related to the sediments that the water picks up along its path and "the sediments may contain a wide variety of pollutants: metals, hydrocarbons, etc."

The researchers also confirmed that the water collected in the tank does not pose an ecological risk for aquatic ecosystems. So, "it might be possible to use the water in the tank for cleaning purposes or even to discharge it into a nearby river or, after filtering and ensuring the biological quality of the water, to use it for irrigation purposes," explained Madrazo.

Further research is needed

As the researchers pointed out, "With a system like this, you can't solve a big problem. Many systems of this kind would need to be set up in a city to manage the water properly. Although the results obtained in the research are currently very good, more research is needed since the Legazpi system is only five years old and we don't know whether in the future the pavement may become saturated and less efficient. What is more, pavements of this type also need to be maintained." It is therefore important to continue to explore them because local conditions vary greatly.

The researcher revealed that such drainage systems are gaining strength in regulations and laws. However, the regulations still in force do not specify the obligation to set up these drainage systems, but "the tendency is to limit discharge from a newly built plot of land; in other words, so that once a plot of land has been urbanized, it does not discharge more than it did in its natural state. So sustainable urban drainage systems are essential," they said.

This group of researchers is involved in other ongoing studies in the Txominenea neighborhood of Donostia-San Sebastian, for example. If such systems are to thrive, several municipal departments need to be involved, and "that is often one of the main difficulties," the researchers said. However, they were keen to point out that the cycle that water fulfills in nature would need to be restored.